Device for the separation of gas ions



Dec. 29, 1959 A. BIERMAN DEVICE FOR THE SEPARATION OF GAS IONS FiledJune 24, 1957 I 3 Sheets-Sheet 1 AMPLIFIER flkon/ INVENTOR.

Arr-mane Dec. 29, 1959 A. BIERMAN DEVICE FOR THE SEPARATION OF GAS IONSFiled June 24, 1957 3 Sheets-Sheet 2 A Bu k -44 INVENTOR.

Dec. 29, 1959 A. BIERMAN 2,919,343

DEVICE FOR THE SEPARATION OF GAS IONS Filed June 24, 1957 3 Sheets-Sheet3 /?e ON .E/E'E HN INVENTOR.

United States This invention relates to a method and apparatus for theseparation of gas ions and other particles of different mass.

A number of methods have already been suggested for the separation ofgas ions by means of so-called mass spectrographs. Generally speaking,these belong to either of two main categories. In the methods of one ofthese the ions are separated by the different accelerations imparted tothem, in accordance with their mass, by a magnetic field, or acombination of a magnetic field and an electrostatic field, throughwhich the ions are made to pass. These mass-spectrographs require a veryhigh mechanical precision as well as cumbersome and expensive magnets.The methods belonging to the other category use electric fields only andthe particles, which have all the same initial energy and the sameelectric charge but different masses, attain dilferent velocitiesaccording to their different masses, and hence cover different distancesin the same time.

Of the methods of the second category there may be mentioned inparticular the one developed by Bennett (J. Appl. Phys. 21, 143, 1950).According to this method, a beam of ions is shot into an electrostaticfield across several pairs of grids on which an alternating potential ofhigh frequency is impressed. Among the different ions entering the spacebetween each pair of the grids there will be some whose mass, and hencevelocity is such that during the entire time of their stay in that spacethe direction of their flight coincides with that of the impressedalternating field. Of these ions some are said to be in resonance withthe impressed high frequency by which they are accelerated, namely thosewhose passage between each pair of grids occurs during the same phaseinterval of the high-frequency potential. The.

other ions, which have a different mass, are not in resonance with theapplied high'frequency and are. consequently not suflicientlyaccelerated by the action of the fields, or are even retarded.

This method has the drawback that all the ions have to have the sameinitial energy, for which purpose rather complicated apparatus has to beused. A further drawback is that ions of a certain mass are alwaysaccompanied by so-called ghost images or harmonics, i.e. by ions whosemass has a whole-number ratio to the mass of the ions underconsideration, andwhose passage from one grid to the other requires alength of time that happens to have a whole-number ratio to the periodof oscillation of the high-frequency potential, so that they are also inresonance with the latter. According to this method, the ghost imagesare reduced by an increase of the number of pairs of grids which,however, makes the apparatus still more complicated and expensive.

It is the object of the present invention to provide a simpler and moreefiicient method and apparatus for the separation of gas ions ofdifferent mass, which may be used both for measuring the abundance ofthe ions and for their bodily separation for preparative purposes.

atent 2,919,348 Patented Dec. 29, 1959 The invention makes use of whatis known as a potential well, i.e. a symmetrical electrostatic fieldwhose potential is low in the centre and rises symmetrically towardsopposite boundaries. The graphical representation in any plane normal tothe electrodes, of the potential of such a field as a function of, forexample, the distance from the median axis results in a well-likedesign, hence the term potential well.

If an ion is introduced with a certain initial energy into such apotential well, it starts performing harmonic, or very nearly harmonic,oscillations, just as a steel ball will do when dropped alongside thewalls of a vessel whose smooth inner walls are parabolic.

The invention consists of a device for the separation of gas ions ofdilferent mass, comprising means for building up a potential well, meansfor superimposing on the electric field of the potential well analternating electric field of variable high frequency in one or moreselected regions of the potential well, means for shooting ions into thepotential well with a certain inclination towards the central axis ofthe device, whereby a zig-zag motion across the device is imparted tothe ions and means for collecting and/ or measuring the abundance ofthose ions whose zig-zag motion is in resonance with said variable highfrequency and which thus gather enough energy inside the device toenable them to reach the Zone of highest potential therein.

The ions that are not in resonance with the superimposed alternatingfrequency do not gather enough energy to reach the zone of highestpotential, and they collect inside the device.

The invention is applicable not only to gas ions, but also toelectrically charged minute particles of matter produced by an atomizingoperation from the liquid or solid state. For .the sake of simplicitythese are not specially mentioned in this specification and the appendedclaims, and they are deemed to be included in the term gas ions.

The potential constituting the outer boundary of the potential well willhave to have the same sign as the ions to be separated, i.e. positive inthe case of positive ions, and negative in the case of negative ions.

The alternating field can be impressed on different parts of thepotential well. pressed on the high-potential boundaries of the Well,leaving thelow-potential regions unaffected. Alternatively, thealternating field can be impressed on the lowpotential regions of thewell or, if desired, on its ascending regions. alt is also possible toimpress several alternatlilng fields each on a diiferent region of thepotential we A particular convenient case is the application of thehigh-frequency potential to the low-potential region of the well, andthis will be described hereinafter.

. A potential well can be realized in a number of ways. For example, twohollow bodies, e.g. pipes of circular or angular cross-section, can beused. The diameter of the one has to be smaller than that of the other,but otherwise they'have to be of similar profile. The Walls of the pipeof smaller diameters are perforated and form a grid. These pipes areplaced insulatedly one within the other and are connected to the twopoles of 'a DC. source.

The simplest and most convenient way of producing a potential Well is touse two parallel plates between which two grids are disposed at equaldistances from and in parallel to the plates. -1The plates are connectedto the positive pole and the grids to the negative pole, of a DC.source. v

The invention is illustrated, by way of example only, in theaccompanying drawings in which:

For example, it can be im- Fig. 1 is a diagrammatic illustration of theapparatus according to the invention;

Fig. 2 is a perspective view of a set of plates and grids for anapparatus according to the invention;

Fig. 3 is a plan view of another apparatus according to the invention,partly broken open; and

Fig. 4 is a perspective view of the inner parts of the apparatusaccording to Fig. 3 in a larger scale.

The apparatus according to Fig. 1 is adapted for the separation ofpositive ions and comprises two parallel plates, 1, 1', electricallyinterconnected through a resistor 2 which is tapped by a wire 3connected to an amplifier 4. The latter is connected by a wire 5 to thepositive pole of a high-potential D.C. source 6, and an indicator, forexample, in the form of a milliammeter 4-, is connected in parallel withthe amplifier 4, Placed between the plates 1, 1', each at the samedistance from one of the plates and parallel to it, are two planar grids7,7,

electrically interconnected through a resistor 8. The latter is tappedby a Wire 9 which leads to the negative pole of the high-potentialsource 6. Between the wires 5 and 9 a resistor 6 is connected inparallel to the DC. source 6. Also connected between the two grids 7,7', and parallel to the resistor 8, is an oscillating circuit whosefrequency can be varied arbitrarily and which includes the terminals 10.

At one end of plate 1 there is provided an ion source 11 with focussingmeans 12 and an accelerator 13, which latter is connected to wire 9 andhence to the negative pole of the DC. source 6, whereas the potentialsfor the ion source 11 and focusser 12 are drawn off from two differentpoints of the resistor 6'. Thus there exists a drop of potential fromthe ion source 11 to the focusser 12 and from the latter to accelerator13. The drop of potential, and consequently the acceleration imparted tothe positive ions, is dependent on the magnitude of the DC. potentialgenerated by the source 6 as well as on the location of the points atwhichv the resistor 6' is tapped. It can, therefore, be adjusted at willby variation of the magnitude of the potential generated by the source6, and/ or by the appropriate location of the points at which theresistor 6 is tapped. The ion source, focussing device and acceleratorare so disposed that they can deliver into the space between the grids,a beam of gas ions in a direction other than normal to the grids andplates. Owing to this inclination, the ions receive a velocity componentin the direction of the axis of the well. The electric energy requiredfor the ion source is equally supplied by the DC. source and is afraction of the total potential of the latter.

At the top of one of the grids 7' a plate 14 is provided for collectingundesired ions, as will be explained below.

This apparatus works as follows:

Let it be assumed that the accelerator 13 emits into the evacuatedinterior of the apparatus a beam consisting of a mixture of positivelycharged ions of different masses, and the plates 1, 1 are chargedpositively. In the absence of a super-imposed high-frequency, all theions, owing to their initial energy, will proceed towards the plate 1'.Their velocities are, however, difierent owing to their differentmasses. The path of the ions isindicated by the arrow. Their velocitiesdiminish more and more as they approach the plate 1, and the ions arefinally repelled by the electric field existing between the grid 7 andplate 1. This repulsion deflects the ions obliquely back towards theplate 1 where they are again similarly deflected. This Zig-zag movementin sweeps continues until the ions eventually hit plate 14. The timeelapsing between consecutive deflections depends on the velocity of eachion, i.e. on its mass. This time will hereinafter be referred to assweeping time.

Suppose now that-the grids are connected to a highfrequency oscillatingcircuit whose oscillating potential is super-imposed on the two grids.As stated above, the frequency of the oscillating circuit is variable.The circuit can, therefore, be so tuned that its frequency is inresonance with the zig-zag movement of a selected kind of ion of thebeam. This means that each cycle of the superimposed frequencysubstantially corresponds to the oscillating period of the ion, i.e. tothe aggregate time of two consecutive sweeps back and forth across theapparatus. Accordingly, when an ion of the selected kind crosses thespace between the grids for the first time during a phase interval ofthe superimposed oscillating frequency which gives rise to theacceleration of the ion, this ion will be accelerated also during allits other passages between the grids. Its energy is thereby increased sothat the ion approaches the one plate 1 (or 1') slightly more than itdid the opposite plate 1' (or 1), at the end of the preceding sweep, asa consequence of which this ion, in contrast to the ions not inresonance with the superimposed oscillating frequency, gradually buildsup sufiicient energy to hit one of the plates 1 or 1'. In the drawing,the path of the selected kind of ion is indicated by a full line whereasthe paths of some other ions are indicated in dashed lines.

It has been found that although the selected ions con tinually build upenergy, their sweeping times are substantially not altered from reversalto reversal. On the one hand, the time of passage between the two gridsis shortened from sweep to sweep owing to the increasing velocity of theions, but on the other hand, the time they dwell in the spaces betweeneach grid and the coordinated plate 1 increases from sweep to sweepowing to the lengthening of their paths within these spaces. The netresult is that the sweeping time remains constant within very narrowlimits irrespective of the variations of the energy of the particles.Similarly the sweeping time is not substantially affected by theenergy-spread of ions emitted by the source of ions. This isparticularly true if the apparatus is so designed, and the conditionsare so chosen, that during the particular sweep when the ionvelocitycomponent in the direction perpendicular to the grids has attained thegeometrical mean between the initial and end values, the ions spendsubstantially equal times in their passage from grid to grid and in thetwo spaces between the grids and the coordinated plates.

At the place where the ions reach the plate 1 and/ or 1 the latter canbe provided with a window and a cage for collecting'the selected ions.Instead, the ions may be made to impinge on the plate 1 or 1' wherebythe charge of the latter is increased. This increase can be measuredwith the aid of the amplifier 4.

All the undesired ions move at such velocities that their sweeping timesare not in resonance with the oscillations of the oscillatory circuit.Consequently, they do not build up suflicient energy to reach eitherplate 1 or 1', and their path is eventually interrupted by the plate 14.

It has been found that the ions separated by the method'according to theinvention are free fro'm the ghost images referred to above. This canpossibly be explained by the fact that when a mass m is in resonancewith the high-frequency, the mass m =(2Z+l) m (Z being an integer) willequally be in resonance, but the energy built up by the latter mass isonly the (2Z-I-l)th part of the energy built up by the first mass. Thisenergy does not suffice for the ions having the masses corresponding tothe above formula to reach plate 1 or plate 1' in spite of the fact thattheir sweeping time is equally in resonance with'the high frequency.

Where two or more kinds of ion of different mass are not only to beseparated, but all of them are to be collected or detected, thefrequency of the superimposed oscillations has to be varied during theoperation so that it is in resonance first with the one kind of ion andthen with the other, or with the several others in succession.

Ina'n advantageous embodiment of the invention the plates 1, 1 are madeconcave and face each other symmetrically with their concavities. Suchan arrangement is diagrammatically illustrated in Fig. 2. The somewhatconcave plates 1, 1' are connected to the positive pole of the D.C.source and thus form the positive upper boundaries of the potentialwell. Between the plates 1, 1 and at equal distance from them there aredisposed the grids 7, 7. The electrical connections and the remainingparts of the arrangement of the apparatus, which have not been shown inFig. 2 will be similar to those illustrated in Fig. l, or in Figs. 3 and4 which are described below.

The advantage of this embodiment lies in the fact that the force of theelectrostatic field formed between plates 1 and grid 7 and plate 1' andgrid 7, respectively, has components directed towards the horizontalmedian plane of the device, whereby the ions are focussed to a certaindegree towards that plane and their random scattering is prevented orminimized.

Figs. 3 and 4 illustrate in greater detail a device according to theinvention designed only for the determination of the abundance of ionsin gaseous mixtures. This device comprises a tube 1, e.g. of glass,composed of a main part 2, an oblique side arm 3 and a tubularconnection 4 for a hose leading to a vacuum pump. Both the arm 3 and theconnection 4 can be sealed hermetically. The main part 2 of the tube 1houses the two plates 5 and 5' which are U-shaped and whose rims 6, 6face each other across the inner space of the tube. The ends of theplates are interconnected by transverse members 7, S of insulatingmaterial, to which they are screwed.

In parallel to plates 5, 5' and at a certain distance from them thereare disposed grids 9 and 9' which are constituted each by a set ofparallel wires. One end of these wires is anchored in the member 8. Theopposite ends of the grid wires 9 are fixed to a plate 10 of conductingmaterial mounted on member 7, whereas the corresponding ends of the gridwires 9 extend without contact through the plate 10. The latter is acollector for the not-selected ions and corresponds to the plate 14 ofFig. 1.

The plates 5, 5' are provided at one end with terminals 11, 11, and atthe opposite end with terminals 12, 12'. Either of these pairs ofterminals can serve for connection to the positive pole of a D.C.source, as in Fig. 1. Similarly the grids 9, 9 are provided at one endwith terminals 13, 13, and at the opposite end with terminals 14, 14'.Either pair of these terminals may serve for connection to the negativepole of the D.C. source and also for connection to the high-frequencyA.C. potential.

Through plate 5' there extends a sleeve 15 which supports anion-producing unit 16, e.g. a gas discharge tube producing positive raysand being associated with focus sing and accelerating devices. Suchunits are known and need, therefore, not be described in detail. Unit 16is provided with an inlet 17 for the introduction of the gas which is tobe ionized. The sleeve 15 may be made, for example, from metal and bewelded to plate 5. It will then be necessary to interpose a layer ofinsulating material (not shown) between the sleeve and unit 16.

In operation the device has first to be evacuated through the connection4, then the gas mixture to be ionized is introduced through inlet 17. Ifdesired, the interior of the device may be swept several times with thegas or gas mixture under investigation before the actual beginning ofthe operation for the removal of all foreign molecules. The degree ofvacuum has to be chosen so that collisions between the ions andnot-ionized molecules are avoided as far as possible.

The following two examples will illustrate the invention:

Example 1 In order to separate and collect deuterium (m=2) fromparticles of other masses one proceeds as follows:

A beam consisting of a mixture of ions of deuterium and the other kindsin question is shot into the apparatus with an aperture angle of 14 anda mean inclination of 17 with regard to a plane normal to plate 1 ofFig. l in the vicinity of the accelerator. The length of the grids is6.1 cm., the distance between each grid and the corresponding near plate1 cm., and the distance between the grids 2.8 cm. The D.C. potentialapplied to the plates is 1000 volts, the D.C. potential at the grids is0 volt and the initial energy of the ions entering the apparatus is 500electron volts. The grids are connected with an oscillating circuitwhose frequency is 2.3 mc./sec. with a potential peak value of 200volts. After 3-8 sweeps of the ions through the apparatus,'the exactvalue being dependent on the inclination at which the particular ion isshot into the apparatus, the deuterium ions build up enough energy toreach the plate 1 or 1' where they can be either collected or measured.In this manner pure deuterium is isolated.

Example 2 For the separation of particles of mass 20 from particles ofany other possible masses the arrangement is as follows:

Length of grids 37 cm.; peak value of superimposed alternating potential22.5 volts; frequency of superimposed alternating potential 0.7mc./sec.; aperture 7; mean inclination 13 /2". The remaining conditionsare the same as in Example 1.

It is found that after 27-45 sweeps, the exact value being dependent onthe inclination at which the particular ion was shot into the apparatus,the particles of mass 20 are obtained in a pure state.

I claim:

1. A device for the separation of gas ions comprising two electrodeplates facing each other with their longitudinal axes in parallel, onepair of grids placed between said electrode plates at equal distancesfrom the latter, a source of D.C. current, said plates being connectedto one pole of said D.C. source and said grids being connected toanother pole of said D.C. source thus forming a potential well, meansfor superimposing on the electric field of said potential well analternating electric field of variable high frequency in at least oneselected region of the potential well, means for shooting into saidpotential well ions with a certain inclination towards a central axis ofthe device midway between said parallel longitudinal axes of the plates,thereby imparting to the ions a zig-zag motion across the device, andmeans for separately receiving those ions whose zig-zag motion is inresonance with said variable high frequency.

2. A device for the separation of gas ions as in claim 1; wherein bothsaid electrode plates and said grids are planar and parallel to eachother.

3. A device for the separation of gas ions as in claim 1; wherein saidelectrode plates are concave and disposed symmetrically in relation to amedian plane extending through said central axis perpendicular to thegrids, said plates having their concavities facing each other.

References Cited in the file of this patent UNITED STATES PATENTS

1. A DEVICE FOR THE SEPARATION OF GAS IONS COMPRISING TWO ELECTRODEPLATES FACING EACH OTHER WITH THEIR LONGITUDINAL AXES IN PARALLEL, ONEPAIR OF GRIDS PLACED BETWEEN SAID ELECTRODE PLATES AT EQUAL DISTANCEFROM THE LATTER, A SOURCE OF D.C. SOURCE AND SAID GRIDS BEING CONONEPOLE OF SAID D.D. SOURCE AND SAID GRIBS BEING CONNECTED TO ANOTHER POLEOF SAID D.C. SOURCE THUS FORMING A POTENTIAL WELL, MEANS FORSUPERIMPOSING ON THE ELECTRIC FIELD OF SAID POTENTIAL WELL ANALTERNATING ELECTRIC FIELD OF VARIABLE HIGH FREQUENCY IN AT LEAS ONESELECTED REGION OF THE POTENTIAL WELL, MEANS FOR SHOOTING INTO SAIDPOTENTIAL WELL IONS WITH A CERTAIN INCLINATION TOWARDS A CENTRAL AXIS OFTHE DEVICE MIDWAY BETWEEN SAID PARALLEL LONGITUDINAL AXES OF THE PLATES,THEREBY IMPARTING TO THE IONS OF ZIG-ZAG MOTION ACROSS THE DEVICE, ANDMEANS FOR SEPARATELY RECEIVING THOSE IONS WHOSE ZIG-ZAG MOTION IS INRESONANCE WITH SAID VARIABLE HIGH FREQWUENCY.